1. Field of the Invention
The invention relates to a power supply arrangement having a first input for connecting to a first power source, at least one second input for connecting to at least one second power source and an output for connecting to a load, the arrangement comprising at least two paralleled inverter modules supplied by said at least one second input and a static switch for switching a connection from said first power source to said load. The invention further relates to an uninterruptible modular power supply system comprising such a power supply arrangement as well as to an inverter module for such a power supply arrangement.
2. Description of the Related Art
Uninterruptible power supplies (UPS) are widely used. They provide an interface between a standard power source (such as AC mains) and sensitive loads (computer systems, security equipment, instrumentation etc.). The uninterruptible power supply comprises an alternate power source which is usually a DC power source (e.g. rectifiers with backup batteries). Inverters are employed for generating an AC output current from the DC input current by recomposing a regulated and continuous sine-wave output. Usual inverters comprise a DC/AC-converter, a regulation system and an output filter.
Often, at least two paralleled inverters are employed for converting the DC input to the AC output utilizable by the critical load in the case of a failure of the standard power source. Providing a plurality of inverters not only allows for higher loads but a certain redundancy may be provided such that in the event an inverter module fails other units may pick up its share and guarantee for uninterrupted supply.
It is further known that the inverters are comprised in inverter modules, that may be easily replaced in the case of defects, or further modules may be easily added if the load is increased or if a higher redundancy level is desired.
In a so-called off line topology, the primary (standard) power source (e.g. AC mains) as well as the inverter group (consisting of the plurality of inverter modules) are connected to inputs of a static switch. The output of the switch is connected to the critical load to be supplied by the power supply arrangement. The static switch (or static bypass switch) is a power-electronics device that allows for ultra-fast switching by employing suitable electronic switching devices (instead of slow mechanical parts). In normal use the static switch is in a position where the load is directly connected to the primary power source (off line mode). As soon as a power failure is detected (i.e. the standard power source fails) the static switch switches to the alternate power source (on line mode). Due to the ultra-fast switching device this change of power source is possible without interruption of the power supply for the critical load.
In the context of these existing modular inverter architectures the critical load is always connected to the energy source via the static switch, in off line as well as in on line mode. Therefore, if the static switch fails the power supply for the critical load is interrupted. This susceptibility to single point failures of the static switch constitutes a major drawback or the known architecture. Such single point failures may be caused by a number of reasons such as microcontroller resets, defects of the quartz clock circuit for the microcontroller or the microcontroller itself, defects in the circuit, which may short circuit the auxiliary supply, a defect of the auxiliary supply or many other reasons. Additionally, a cooling system for the static switch is required, either based on convection cooling or employing redundant fans.
In principle, these drawbacks may be avoided by building a static switch with full redundancy. However, this leads to a major impact on the cost of the arrangement.
Furthermore, even with a switch with full redundancy there is the additional drawback that the static switch has to be replaced by using a manual bypass if maintenance procedures are to be carried out. The manual bypass switch allows for supplying the load via direct connection to the standard power source. If this replacement is not carried out correctly by the maintenance staff the power supply for the critical load will be interrupted, i.e. the reliability of the system depends on the maintenance instructions and human skills and attention. This calls for complicated and lengthy prescribed maintenance procedures in order to minimize the risk of failures. However, human errors can never be completely prevented and are therefore a permanent threat to the reliability of the known systems.